Add workflow for automatically merging Steward's PRs. (#334)

Signed-off-by: Ignacio Lucero <ignacio.lucero@moia.io>

Author: houcros

.github/workflows/automatic-merge.yaml

@@ -0,0 +1,17 @@
+name: Scala Steward automatic merge
+on: pull_request
+
+permissions:
+  contents: write
+  pull-requests: write
+
+jobs:
+  scala-steward-auto-merge:
+    runs-on: ubuntu-latest
+    if: ${{ github.actor == 'scala-steward-for-moia[bot]' }}
+    steps:
+      - name: Enable auto-merge for Scala Steward PRs
+        run: gh pr merge --auto --squash "$PR_URL"
+        env:
+          PR_URL: ${{github.event.pull_request.html_url}}
+          GITHUB_TOKEN: ${{secrets.GITHUB_TOKEN}}